The elbow is formed at the meeting of the distal end of the humerus and the proximal ends of the radius and ulna. In the elbow, the head of radius articulates with the capitellum of the humerus and radial notch of the ulna. The trochlear notch of the ulna articulates with the trochlea of the humerus. When the forearm is in extreme flexion, the head of the radius fits into the radial fossa located just above the capitellum. The lateral epicondyle is found on the lateral aspect of the humerus just above the capitellum. The lateral epicondyle serves as the insertion for one portion of the supinator muscle which attaches at about the proximal third of the radius.
Along their lengths the radius and ulna are joined by the interosseous ligament, also known as the interosseous membrane. The interosseous ligament has a fibrous structure that is oriented on a bias relative to the axis of the forearm. This ligamentous interconnection between the radius and the ulna serves to transfer loads at the wrist from the radius to the ulna. At the wrist, the distal end of the radius is a larger and more robust structure than the head of the ulna. Thus, at the wrist a large share of the load carried by the hand is transferred to the radius. However, at the elbow, the ulna is substantially larger than the head of the radius. The interosseous ligament serves to transfer loads from the radius to the ulna so that the load is substantially equalized at the elbow or so the load is carried more by the ulna than the radius. The loading distribution between the radius and the ulna at the humeral articulation is dynamic, varying with the position and motion of the forearm and hand.
The radial head is an important component of normal elbow and forearm function. The radial head contributes to both the radiocapitellar and proximal radioulnar joints.
The radial head makes a 12 to 15 degree lateral angle to the radial shaft. This lateral angle is away from the radial tuberosity which is located distal to the radial head on the medial side of the radius. Various ligaments about the radial head provide important soft tissue support and are essential to elbow stability. The stress distribution at the elbow varies in pronation and supination but averages about 60 percent to the radiohumeral joint and about 40 percent at the ulnar humeral articulation.
The radial head can be damaged in many ways. One common source of damage the head of the radius is a fall onto an outstretched hand. Historically, if the radial head was damaged so severely as to be impossible to repair with the use of bone screws or other internal fixation structures, the radial head was resected and the elbow closed. Radial head resection was sometimes necessary due to fracture, osteochondrosis or secondary arthritis. Unfortunately, radial head resection tends to cause significant adverse effects on elbow and forearm function.
Resection of the radial head tends to cause persistent elbow instability following elbow fracture, dislocation, rotational instability injuries and medial lateral translation injuries. Excision of the radial head can also cause forearm axial instability, particularly if the remaining stabilizing structures have been compromised. Since the radial head acts as a secondary stabilizer to the elbow joint, once the radial head is removed the soft tissue stabilizers including the collateral ligaments, the interosseous membrane and the articular surfaces of the radial ulnar joints may be compromised and the instability of the joint tends to increase. Thus, replacement of the radial head with an implant has been embraced as a way to attempt to restore anatomic normalcy and functional usefulness to the elbow and forearm.
Radial head implants exist in the art. Typically, radial head implants are available in multiple sizes to approximate anthropomorphic differences in radial head size in different patients. A radial head implant generally will include a radial head component and a stem component. The radial head component is designed to anatomically articulate with the convexity of the capitellum for an anatomic joint surface contact area. The circumference of the head matches the normal radioulnar joint articulation, preserves the annular ligament and minimizes release for exposure of important lateral ulnar collateral ligament. The radial head and radial stem may be joined together by a Morse tapered protrusion that is adapted to interface with a Morse tapered cavity. Generally, the Morse tapered cavity is located in the head which allows placement of the radial head onto the protrusion on the radial stem.
A typical radial prosthesis has a stem configured for implantation into the intramedullary canal of the proximal radius. Some types of radial head implant include a limited ball joint articulation between the radial head component and the stem component. These are sometimes known as bipolar implants. In this type of implant, the concave articular surface of the radial head anatomically articulates with the convexity of the capitellum while the radial head articulates with the stem of the implant. This allows the use of a radial head implant in situations where the radial head may be damaged in such a way as to make implantation of a fixed radial head implant difficult or impossible. Use of a bipolar radial head implant allows for some adjustment of the head of the implant with the capitellum once the implant is in place. The articulation between the head and the stem allows for the correction of angular alignment deformities between the radius and the capitellum. The ball and socket design of this type of radial head implant allows radial-capitellar contact to be maintained through a functional range of flexion and forearm rotation.
Unfortunately, some falls onto an outstretched hand will also damage the capitellum of the humerus. In addition, the capitellum is sometimes to subject to arthritic degeneration or increased wear after placement of a radial head implant.
As discussed above, sometimes resection of the radial head is performed without an implant. Resection of the radial head leaves the patient with a so-called ulna plus variation which tends to lead to wrist pain which, in turn, can be debilitating. For these reasons, it would be beneficial to the patient to be able to surgically replace and restore the capitellum as well as the radial head.
Traumatic injury of the radioulnar joint is common. Injuries to the radioulnar joint include radial head fractures and radial head fractures associated with ligament injuries; combined proximal ulna fracture with radial dislocation or fracture; radial head fracture associated with dislocation of the elbow; and forearm and elbow injuries in combination. Forearm and elbow injuries include radial head fracture combined with interosseous membrane disruption, the so-called Essex-Lopresti lesion. In the case of an Essex-Lopresti lesion or interosseous ligament tear, the transfer of loads from the hand and then from the radius to the ulna is compromised. This lack of transfer from the radius to the ulna creates an increased load at the radial head where it meets the capitellum which increases wear on the capitellum as well as the radial head.
Thus, it would be beneficial to the orthopedic arts to have an implant available to surgically repair damage to the capitellum as well as to the head of the radius.